Digital filter bank and spectrum analysis and synthesis are extremely important in a variety of fields, including voice and data communication. The IRIDIUM.TM. Satellite Cellular Communications System, for example, calls for use of an L-band modem link using a 24-channel frequency division multiple access (FDMA) signal. The FDMA signal must be channelized to provide for proper interfacing to single-channel modulators and demodulators. At the transmitter, the baseband signals must be filtered and translated to the appropriate frequencies for summation into an overall FDMA signal (i.e., multiplexed). In the receiver, the various channels of the FDMA input signal must be separated by filtering and translated to baseband (i.e., demultiplexed) for input to a bank of demodulators.
Taking a brute force approach to generating and separating FDMA signals requires a channel filter and local oscillator for each FDMA channel. This approach, however, results in a large size and weight for a large number of channels, which is intolerable for a space-borne system.
Various techniques have been described for digital filter bank and spectrum analysis and synthesis. The text Multirate Digital Signal Processing, by Ronald E. Crochiere and Lawrence R. Rabiner, published in 1983 by Prentice-Hall, Inc., Englewood Cliffs, N.J., develops such techniques for several cases, including a complex filter bank and single-sideband filter bank. The Crochiere and Rabiner technique is not applied to teach a hardware-efficient method for generating and separating FDMA signals, however, such as are required for the IRIDIUM.TM. System.
Thus, what is needed is a method for FDMA signal multiplexing and demultiplexing which reduces filter computation and storage requirements, frequency translation computations, and minimizes size and weight. Specifically, the method should allow the transmission and reception of real signals while manipulating complex baseband signals (I and Q data of a quadrature phase shift key (QPSK) signal).